In some applications utilizing a power amplifier, it is desirable to limit the peak voltage that the switching devices of the power amplifier are subjected to. For example, in CMOS devices, the transistor breakdown voltage may be only slightly greater than the supply voltage. Therefore, CMOS devices are not well suited to traditional power amplifier designs, where switching devices are subjected to voltages at least twice the supply voltage.
FIG. 1 is a schematic diagram of a conventional Class E amplifier. As shown, a transistor M1 is connected between ground and an inductor L1 which is connected to a voltage source Vdd. The gate of the transistor M1 is connected to an input signal Vi. The connection of the transistor M1 and the inductor L1 forms a node labeled Vd. The switching device M1, as well as other switching devices described may be comprised of any suitable switching devices, for example, MOSFETs or other transistor types. A capacitor C1 is connected between Vd and ground. The amplifier includes a transformation network consisting of inductor L2 and capacitor C2. The capacitor C2 is connected to a load RL at output node Vo.
FIG. 2 is a timing diagram illustrating the input signal Vi and the resulting voltage at Vd. As shown, the input signal Vi is a square wave signal switching between ground and Vdd. When the input signal Vi is high (Vdd), the transistor M1 is turned on, holding Vd to ground. When the input signal Vi transitions to low, transistor M1 turns off and the voltage at Vd rises above Vdd. During this time, the transistor M1 must sustain this high drain-to-source voltage. After peaking, the voltage at Vd decreases until it reaches ground. In a typical prior art Class E design, this peak voltage is approximately 3.6 Vdd. Although the peak voltage can be reduced slightly, it can not be decreased below about 2.5 Vdd since the average voltage at Vd must equal Vdd. Designs such as that shown in FIG. 1 are not well suited to certain device technologies, such as CMOS, where transistor breakdown voltages are only slightly higher than the supply voltage.
It can therefore be seen that there is a need for amplifier designs where the peak voltages applied to the transistors of the amplifier are reduced so that they are below the transistor breakdown voltages of the devices being used to implement the design.
Another problem relating to amplifiers relates to the use of differential circuits. It is difficult to perform differential-to-single-ended conversion when a single ended load is required with high efficiency. Therefore, there is a need for improved differential-to-single-ended conversion designs.
Another problem with differential RF power amplifiers relates to stabilization of both the differential and common modes. Typically, all power amplifier modes are required to be stable over a specified set of loads and operating conditions. There is a need for techniques which stabilize power amplifiers over these conditions without compromising efficiency.